<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        html {
            font-size: 16px;
        }
        
        .box {
            font-size: 12px;
            margin: 100px auto;
            width: 20em;
            height: 12em;
            background-color: chartreuse;
        }
    </style>
</head>

<body>
    <div class="box"></div>
</body>

</html>